1. Field of the Invention
The present invention is concerned with three-way conversion (“TWC”) catalyst compositions which are effective for the suppression of hydrogen sulfide. The catalyst compositions may be used in the “close-coupled” or “medium-coupled” mode. Optionally, a downstream underfloor catalytic converter may also be present.
2. Description of Related Art
The use of TWC catalyst compositions to treat the exhaust gases emanating from internal combustion engines, e.g., automobile engines, is well known. Such catalyst compositions are polyfunctional in that they simultaneously treat unburned hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gases.
“Close-coupled” catalysts are known in the prior art and are generally defined as located in the engine compartment, typically less than one foot, more typically less than six inches from, and commonly attached directly to, the outlet of the exhaust manifold. “Medium-coupled” catalysts are also known in the prior art and are generally defined as located (downstream of any close-coupled catalyst)usually not more than about twenty-four, typically eighteen, inches from the outlet of the exhaust manifold. Underfloor catalytic converters are also known in the prior art and are located (downstream of any close-coupled and/or medium-coupled catalysts)under the floor of the vehicle adjacent to or in combination with the vehicle's muffler.
In many cases, TWC catalysts convert sulfur compounds contained in the exhaust gases into hydrogen sulfide which is quite toxic and is well known for its extremely disagreeable odor, even in minute quantities. Methods of suppressing the formation of hydrogen sulfide by including cobalt, nickel, iron, manganese or rhenium in the TWC catalyst compositions are known. For example, U.S. Pat. No. 4,552,733 discloses TWC catalyst compositions comprising one or more platinum-group metals dispersed on a gamma alumina support wherein abase metal oxide is dispersed with the platinum-group metal. The base metal oxide may be nickel, iron or manganese. The catalyst described in the '733 patent is stated to have three-way capability without producing appreciable quantities of hydrogen sulfide, sulfur trioxide or sulfuric acid. See also U.S. Pat. No. 4,780,447 which discloses a catalyst which is capable of controlling HC, CO and NOx as well as H2S in emissions from the tailpipe of catalytic converter-equipped automobiles. The use of the oxides of nickel and/or iron is disclosed as a hydrogen sulfide gettering-type of compound.
There is concern in Europe, however, that cobalt and nickel compounds may be carcinogens and therefore, automotive manufacturers are reluctant to use catalysts containing such metals. In other cases, the net effect of including iron, manganese or rhenium in the TWC catalyst is considered negative by automotive manufacturers even though significant positive effects are obtained by their inclusion.
U.S. Pat. No. 5,196,390 overcomes many of the problems alluded to above by providing TWC catalyst compositions which suppress H2S formation. The TWC catalysts provided by the '390 patent involve the incorporation of a nickel, manganese or iron oxide into an undercoat layer disposed on a substrate, such as cordierite. A topcoat overlying the undercoat is comprised of a platinum-group metal, i.e., platinum, palladium, rhodium or mixtures of two or more of the foregoing platinum-group metals.
Although the '390 patent represented a significant advance in TWC catalysts having hydrogen sulfide-suppression properties, automotive manufacturers are nevertheless still reluctant to use TWC catalyst compositions containing a nickel, manganese or iron oxide.